Modern wireless communications devices, such as cellular telephones, are held to ever-higher performance standards. Transmissions must be clear and undistorted, and the battery in the devices must be small and have a long life. In order to meet these consumer requirements, wireless telephone designers have moved away from using traditional silicon-based bipolar transistors as power amplifiers and toward using more exotic transistors, such as heterojunction bipolar transistors (“HBTs”) made of aluminum-gallium-arsenide/gallium-arsenide (“AlGaAs/GaAs”), indium-gallium-phosphide/gallium-arsenide (“InGaP/GaAs”), InGaP/GaAsN, and double heterojunction bipolar transistor (DHBT) in an non-exhaustive listing. Such HBTs provide improved power efficiency and linearity, thus enabling cellular phones to achieve longer battery life and better signal characteristics for voice and data.
Of course, an HBT, like a bipolar junction transistor (“BJT”), requires a direct-current (“DC”) bias signal (comprising both a voltage and a current) to be applied to the input terminal to establish its operating point. (The operating point of a transistor may be defined as the point on the transistor's characteristic curves at which the transistor will operate in the absence of an input signal. See, e.g., John Markus, Electronics Dictionary 445 (4th ed. 1979).) Because changes in the DC bias signal affect the operating point of the HBT (and thus adversely affect the linearity of the amplifier), the DC bias signal must be stable and unaffected by variations in temperature or in the power supply voltage. Typically a “bias circuit” generates such a DC bias signal.
In addition, it is desirable to use amplifying circuits based on the HBT, like the BJT, as off-the-shelf gain blocks for implementing particular device designs. An early and still popular design for such gain blocks is based on the Darlington transistor, which is also known as the Darlington pair. The Darlington arrangement is named after Sydney Darlington of Bell Labs who first proposed the arrangement and received the U.S. Pat. No. 2,663,806 describing it on Dec. 22, 1953. The Darlington arrangement provides high current gain by connecting two (or more) transistors in a cascade configuration with the collectors of the transistors connected together and the emitter of one transistor connected to the base of the other transistor. Resistors connected across the base-emitter of each transistor allow independent design of bias current and reduce the time to turn OFF the conducting pair. In principle more than two transistors may be employed in a Darlington arrangement although typically two transistors are used.
An example of a Darlington pair based gain block is the HBT technology based MINI-CIRCUITS® ERA series of amplifiers for wide-band microwave (from about 50 to 8000 MHz) amplification. These amplifiers are four terminal devices with two of the terminals (pins 2 and 4) connected to ground. Biasing is through the output terminal, which is connected to the supply voltage through a biasing resistor and a recommended RF wide-band choke. This arrangement reflects the current biased nature of the device making it more sensitive to current than voltage fluctuations. Hence a voltage source is approximated as a current source with the use of the biasing resistor for adequate biasing. In addition, the supply is isolated from stray signals by a suitable bypass capacitor. The input (at terminal labeled as pin 1) and output (at terminal labeled as pin 3) signals are provided via respective DC blocking capacitors.
Proper biasing of the aforementioned ERA and similar devices requires an adequately regulated supply voltage. In order to maintain constant operating conditions the recommended supply voltage for such devices is, typically, significantly greater than the device voltage. The device voltage, i.e., the voltage across the device, is modeled to be dependent on both the bias current and the temperature such that it increases with increase in bias current and decreases with increase in device temperature. The use of a large biasing resistor proportionally reduces the variation in the bias current due to temperature or voltage changes since most of the change is in the voltage across the biasing resistor. Alternative temperature compensated biasing of ERA like devices includes a combination of a linear positive-temperature-coefficient thermistor in parallel with a regular resistor such that the decrease in device voltage with temperature is offset by an increase in the resistance of this combination, which is placed in series with the biasing resistor.
The approximate current source configurations described above necessarily result in significant resistive losses resulting in low efficiency. For instance, in the described resistive biasing scheme increasing the supply voltage relative to the specified device voltage improves stability by reducing voltage fluctuations at the device due to fluctuations in the supply voltage. However, this increase in stability is coupled with a reduction in the overall efficiency due to increased resistive losses. This tradeoff results in a preferred value for the supply voltage at about twice the device voltage value. In the case of typical Darlington amplifiers, the expected efficiency is about 15% to 25%.